Seawater desalination is no easy task. Traditionally, vacuum
distillation techniques have been used, and this process requires a great deal
of energy. More recently, reverse osmosis facilities have come online. These
facilities use a special permeable membrane to separate salts and minerals from
liquid water. As of January, there are more than 13,000 desalination plants
pumping out over 12 billion gallons of water a day.

While cogeneration facilities have helped to cut down the cost of vacuum
distillation and reverse osmosis plants are marginally more efficient in terms
of energy used, work in the unrelated field of biology and molecular transport
systems may promise an even better type of filtering membrane for osmosis
systems.

Researchers at the Lawrence Livermore National Laboratory, while studying the
process of molecular
transportation across cell membranes, have come up with a new type of
filter. The filter mimics the properties of cell membranes and is especially
efficient in the transport and filtration of water molecules. Their work is
based on -- you guessed it -- carbon nanotubes.

What makes these CNT-based filters so efficient is their hydrophobic
properties, or the way they repel water molecules. Water moves through the CNT
channels in a membrane much more quickly than in conventional filters because
the molecules do not stick to the sides of CNTs. Instead, they stream through
them unhindered, like a bullet train.

After this discovery, the researchers tested their artificial membrane for
desalination efficiency. They found that the 1.6nm diameter tubes successfully
prevented the ions that make up salts from traversing while letting the water
molecules flow freely. The mechanism for this selectiveness lies in the small
diameter of the channel along with the charge held at the end of the nanotubes.

Aleksander Noy, a senior member of the LLNL research team explains, “while
carbon nanotube membranes can achieve similar rejection as membranes with
similarly sized pores, they will provide considerably higher permeability,
which makes them potentially much more efficient than the current generation of
membranes.”

While the LLNL findings promise a great deal for desalination and other
processes where similar filtering is used, some refinement will still be
required. The ability to construct membranes of different materials and control
the charge and diameter of the nanotube pores will weigh heavily into their
efficiency and usefulness in different applications.

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